The development and maintenance of the vertebrate nervous system depends on specific proteins, termed neurotrophic factors, originally defined by their ability to support the survival of neuronal populations (Snider and Johnson, 1989, Ann. Neurol. 26:489). Neurotrophic factors have also been implicated in processes involving the proliferation and differentiation of neurons (Cattaneo and McKay, 1990, Nature 347: 762-765; Lindsay and Harmar, 1989, Nature 337: 362-364), and they may play additional, thus far unexplored, roles both within as well as outside of the nervous system. Brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) have recently been molecularly cloned and shown to be structurally related to the prototypical neuronal survival molecule, nerve growth factor (NGF). (Leibrock et al., 1989, Nature 341:149-152; Hohn et al., 1990, Nature 344:339-341; Maisonpierre et al., 1990a, Science 247:1446-1451; Rosenthal et al., 1990, Neuron 4:767-773; Ernfors et al., 1990, Proc. Natl. Acad. Sci. U.S.A. 87:5454-5458; Jones and Reichardt, 1990, Proc. Natl. Acad. Sci. U.S.A. 87:8060-8064). These three related factors (designated "neurotrophins") do not display any structural homology to a fourth neurotrophic factor, ciliary neurotrophic factor (CNTF). (Lin et al., 1989, Science 246:1023-1025; Stockli et al., 1989, Nature 342:920-923).
The receptor and signal transduction pathways utilized by NGF have been extensively studied, in large part due to the availability of a pheochromocytoma cell line (PC12) which differentiates in response to NGF (Greene and Tischler, 1976, Proc. Natl. Acad. Sci. U.S.A. 73:2424). These studies have resulted in the cloning of a transmembrane protein (designated "LNGFR" for low-affinity NGF receptor) which binds NGF with relatively low affinity (Chao et al., 1986, Science 232:518-521; Radeke et al., 1987, Nature 325:593-597). In addition to the LNGFR another protein (designated "HNGFR" for high-affinity NGF receptor), which is involved in forming a higher affinity binding site for NGF, is apparently required to initiate NGF-induced signal transduction (Zimmerman et al., 1978, J. Supramol. Struc. 92:351-361; Sutter et al., 1979 in Transmembrane Signalling (N.Y. Alan Liss) pp. 659-667; Bernd and Greene, 1984, J. Bio. Chem. 259:15509-15516; Hempstead et al., 1989, Science 243:373-375). This HNGFR is phosphorylated on tyrosine in response to NGF, and apparently contains intrinsic tyrosine kinase activity (Meakin and Shooter, 1991a, Neuron 6:153-163). Furthermore, early intermediates in tyrosine kinase activated signal cascades, the ERK kinases (also known as the MAP2 kinases), are rapidly activated and phosphorylated on tyrosine in response to NGF. Thus, like many other growth factor responses, NGF signal transduction may be initiated by the activation of a receptor-linked tyrosine kinase.
Recent studies have revealed that the product of the trk proto-oncogene, which resembles a growth factor receptor (i.e., it is a transmembrane protein containing an intracytoplasmic tyrosine kinase domain) for which no ligand had been identified, is rapidly phosphorylated in response to NGF treatment in PC12 cells (Kaplan et al., 1991, Nature 350:156-160; Klein et al., 1991, Cell 65:189-197) and to directly bind NGF with relatively high affinity when expressed in heterologous cells (Klein et al. supra. This finding, together with the restricted neuronal distribution of the trk protein in vivo, suggests that trk may be the component of the HNGFR responsible for initiating NGF signal transduction.
In contrast to the extensive study of NGF receptors and signal transduction pathways, the receptors and signal transduction pathways utilized by the other neurotrophic factors have only recently begun to be explored. However, BDNF appears to bind to the LNGFR with an affinity similar to that of NGF (Rodriguez-Tebar et al., 1990, Neuron 4:487-492). Although both low and high affinity receptors for BDNF exist on neurons responsive to BDNF, the findings that BDNF and NGF act on different neurons and that NGF-responsive neurons do not express high-affinity BDNF receptors suggest that BDNF utilizes a different high affinity receptor than NGF (Rodriguez-Tebar and Barde, 1988, J. Neurosc. 8:3337-3342).
A variety of findings seem to link BDNF and NT-3, while distinguishing both of these neurotrophins from NGF. NT-3 and BDNF (but not NGF) expression displays striking reciprocal relationships during development, with NT-3 being expressed more prominently early and BDNF more prominently late during the development of some of the same brain regions (Maisonpierre et al., 1990, Neuron 5: 501-509). Interestingly, the distribution profiles that BDNF and NT-3 (but not NGF) ultimately achieve in various adult brain regions are quite similar. Id. In peripheral ganglia both BDNF and NT-3 (but not NGF) have their most prominent effects on dorsal root ganglia and nodose ganglia, although NT-3 does seem to have minor effects on sympathetic ganglia (Maisonpierre et al. 1990a, Science 247: 1446-1451)
These findings led to the suggestion that BDNF and NT-3 might in some cases act on the same neuronal populations, and that an early effect of NT-3 on these neurons might be replaced by a later effect of BDNF (Maisonpierre et al., 1990b, Neurons 52:501-509). Furthermore, the finding that BDNF and NT-3 (but not NGF) are the most highly conserved growth factors yet described led to the suggestion that both these factors might be interacting with multiple receptors and that their strict conservation was required to maintain the specificity of their interactions with these multiple receptors.
Klein et al. (1989, EMBO J. 8:3701-3709) reported the isolation of trkB, which encoded a new member of the tyrosine protein kinase family of receptors found to be highly related to the human trk protooncogene (FIG. 8A-D). At the amino acid level, the products of trk and trkB were found to share 57 percent homology in their extracellular regions, including 9 of the 11 cysteines present in trk. This homology was found to increase to 88 percent within their respective tyrosine kinase catalytic domains. In adult mice, trkB was found to be preferentially expressed in brain tissue, although significant levels of trkB RNAs were also observed in lung, muscle, and ovaries. Further, trkB transcripts were detected in mid and late gestation embryos. In situ hybridization analysis of 14 and 18 day old mouse embryos indicated that trkB transcripts were localized in the central and peripheral nervous systems, including brain, spinal cord, spinal and cranial ganglia, paravertebral trunk of the sympathetic nervous system and various innervation pathways, suggesting that the trkB gene product may be a receptor involved in neurogenesis and early neurol development as well as playing a role in the adult nervous system.
In 1990, Klein et al. (Cell 61:647-656) reported that the mouse trkB locus codes for at least two classes of receptor-like molecules, which they designated gp145.sup.trkB and gp95.sup.trkB. These molecules appeared to have identical extracellular and transmembrane domains, but only gp145.sup.trkB was found to contain a long cytoplasmic region that included a catalytic protein kinase domain. TrkB transcripts coding for this protein were observed in the cerebral cortex and the pyramidal cell layer of the hippocampus, whereas transcripts encoding gp95.sup.trkB were found in the ependymal linings of the cerebral ventricles and in the choroid plexus. Further, Middlemas et al. (1991, Mol. Cell. Biol. 11:143-153) reported the existence of two distinct C-terminally truncated receptors which share the complete extracellular region and transmembrane domain with gp145.sup.trkB but which differ from gp145.sup.trkB (hitherto referred to simply as trkB) in their short cytoplasmic tails.